It is customary in digital electronic circuits to transmit parallel existing binary coded digital words with the use of data selectors (multiplexers) serially into a microcomputer. The multiplexer successively connects each of its parallel existing input data lines as a function of control signals to its inputs (address inputs) provided for this with a common output data line. Since the control signals are likewise binary coded, three control signals (address lines) are required to transmit a data word of 8-bit length, and four control signals are necessary to transmit a data word of 16-bit length, in order to be able to control each individual bit to be transmitted.
Frequently, the input circuit and the microcomputer operate asynchronously, that is, they function independently of each other. Thus, the production of measured values and the subsequent processing of these measured values have no firm of permanent time relationship.
A generally known method for the transmission of measured values in the case of asynchronous production and processing of the measured values consists in the fact that the microcomputer is signaled at the occurrence of a new measured value by controlling an interrupt line. The microcomputer then interrupts the running program, takes over the new measured value and subsequently processes the original program again (COMPUTER DESIGN, November 1976, p. 142-143).
However, this method has the disadvantage that an unnecessarily large amount of computing time is used for processing of interrupt programs, even if not all new measured values are needed. Furthermore, the task of wiring technique is relatively difficult and expensive.
It will be appreciated that in an asynchronous transmission system, the calculation of a new measured value and the transmission into the microcomputer are events taking place independent of each other so that erroneous measured values are transmitted when the measured value changes during the serial transmission through the multiplexer into the microcomputer.